<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<div id="app">
  <input v-model="username">
  <!--插值表达式-->
  {{username}}
</div>

<script src="js/vue.js"></script>
<script>

  //1.创建vue的核心对象
  new Vue({
    el:"#app",
    data(){
      return {
        username:""
      }
    }
  });
  /*data:function () {
      return {
        username:""
      }
    }*/


</script>

</body>
</html>